Damping turbine buckets



N0 17, 1959 R. v. m ml. 2,913,221

DAMPING TURBINE BUCKETS Fil ed Dec. 12, 1955 Inventor's.- Robert M/f/fn',

Anthony J. Nerad,

- .by )4! f M 7/7 eir Attorney.

DAMPING TURBINE BUCKETS Robert V. Klint, Vishers Ferry, and Anthony J. Nerad, Alplaus, N.Y., assignors to General Electric Company, a corporation of New York Application December 12, 1955, Serial No. 552,637

Claims. (Cl. 253-77) This invention relates to fluid turbines and more specifically to thevibration damping of turbine buckets in gas and steam turbines.

A serious problem, which is presented in steam and gas turbine operation, is the tendency of turbine buckets to fracture when they are subjected to a vibrational stimulus at or near resonant frequency. During acceleration or deceleration of steam or gas turbines, bucket movement produces vibrational stresses whose amplitude of vibra tion can readily build up to a point where fatigue fractures occurs. Such fractures have occurred in very tough and heat stable metals or alloys from which turbine buckets are fabricated.

It is desirable to provide frictional forces which oppose bucket stresses to reduce fatigue fractures. Although bucket roots on a conventional rotor are in close contact, such a construction fails to produce sufficient frictional engagement between adjacent buckets. The invention of the present application provides a turbine bucket which has its parts in frictional engagement to relieve vibrational stresses within the structure.

Accordingly, it is an object of our invention to provide a new and improved turbine bucket.

It is another object of the invention to provide an improved turbine bucket which will resist fracture when a vibrational stimulus is applied thereto.

It it a further object of the invention to provide a turbine bucket which utilizes frictional forces between variable surface bucket portions to achieve damping.

In carrying out our invention in one form, a turbine bucket is provided with a slit from the bottom of the base through at least a portion of the blade and generally spanwise of the blade to produce adjacent bucket portions which are secured together.

These and various other objects, features, and advan-' tages of the invention will be better understood from the following description taken in connection withthe accompanying drawing in which:

Fig. 1 is a perspective view of a turbine bucket which embodies my invention;

Fig. 2 is a front elevational and partial sectional view of a partially assembled turbine bucket which is shown in Fig. 1; and

Fig. 3 is a front elevational and partial sectional view of a modified turbine bucket which is partially assembled.

In Fig. 1 of the drawing, a turbine bucket is shown which comprises a dovetail base or root with a blade or bucket 11 thereon. A slit 12 is provided in the bucket from the bottom of base 10 through at least a part of blade 11 and generally spanwise thereof to provide adjacent bucket portions 13 and 14. Slit 12 may be described as extending through the thickness of the blade, that is, assuming the blade 11 of Fig. 1 as a rectangle having the dimensions of length, Width and thickness, that the slit 12 extends through the thickness from one side surface to the other. A retaining member, such as, a bolt 15 is positioned in base 10 to adjustably secure United States, Patent 0 2,913,221 1C6 PaitentedNov. 17,1959

variable areas of or surfaces of bucket portions 13 and 14 in contiguous relationship.

As is best shown in Fig. 2, bucket portions 13 and 14 have opposed contact surfaces 16 and 17 which define slit 12. While vibrational damping of the bucket is produced by a slit through base 10, it is desirable to' provide a wedge-shaped slit 12 which extends a substantial distance along blade 11 to increase this damping effect. We have also found that location of the slit at some plane of asymmetry provides additional damping. Bolt 15,

. which is positioned in an aperture 18 within base 10, is

employed to secure varying amounts of surfaces 161 and 17 of bucket portions 13 and 14 together in frictional engagement. Aperture 18 is illustrated as extending through bucket portion 14 without threads on oneside of slit 12 and into portion 13 with threads on the other side of slit 12. Bolt 15 is thereafter slidably inserted n the non-threaded portion of aperture 18 to engage the threaded portion on the other side of the slit 12. Suitable engaging means such, as for example, the head of bolt 15 engages bucket portion 14 or the edge of the opening therein so that rotation of bolt 15 causes a threaded progression of bolt 15 into opening 18 to positively move portions 13 and 14 together for progressive engagement of the surfaces 16 and =17. When a plurality of such buckets are revolved on a rotor, bucket surfaces 16 and 17 rub together to provide frictional forces which reduce vibrations. Thus, vibration amplitude does not increase sufficiently to produce fatigue fractures in the buckets.

In Fig. 3, a modified turbine bucket is shown which comprises a base 10 with blade 11 thereon. The bucket has two adjacent portions or sections 19 and 20 which can be manufactured separately or produced from a onepiece bucket. The portions are then joined together at the blade end by a weld 21. Bucket portions 19 and 20 with opposed surfaces 16 and 17 define a Wedgeshaped slit 12 which may be progressively closed by bolt 15. Such a bucket, which can be fabricated easily, provides desirable damping effects.

As will be apparent to those skilled in the art, the objects of our invention are attained by the use of a turbine bucket with a slit from the bottom of the base through at least a part of the blade and spanwise thereof to produce adjacent bucket surfaces which are adjustably secured together for variable area contact.

While other modifications of this invention and variations of apparatus which may 'be employed within the scope of the invention have not been described, the invention is intended to include all such as may be embraced within the following claims.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A turbine bucket comprising a base, a blade mount ed on said base, said bucket having a Wedge-shaped slit Whose widermost part is at the bottom of said base, said slit being through the thickness of the blade and extending spanwise through the blade a distance less than the length of said blade, said slit providing adjacent bucket portions and contact surfaces, and means in said base cooperating with the bucket portions on opposite sides of said slit to adj-ustably secure said contact surfaces in engagement.

2. A turbine bucket comprising a base, a blade mounted on said base, said bucket having a wedge-shaped slit whose widermost part is at the bottom of said base, said slit being through the thickness of the blade and extending lengthwise through the blade a distance less than the length of said blade, said slit providing adjacent bucket portions and contact surfaces, and means in said base cooperating with the bucket portions on opposite sides of said slit to adjustably secure varying areas of contact of said contact surfaces.

3. The invention as claimed in claim 2 wherein said means includes an opening through one of said bucket portions on one side of said slit, and a threaded opening in the other bucket portion on the other side of said slit, and a threaded member slidable through the said through opening and having means engaging the said one bucket portion, and threadedly engaging the said threaded opening in the other bucket portoin.

' 4. A turbine bucket comprising a base, a blade mounted on said base, said bucket having a slit through the thickness of the blade from the bottom of the base through the whole of said blade, said bucket being joined permanently together at its blade end to provide adjacent bucket portions, the said bucket portions defining a wedge-shaped slit having the widermost portion at the bucket base, and retaining adjustable means in said base to secure varying areas of said bucket portions in frictional engagement.

5. The invention as claimed in claim 4 wherein said adjustable retaining means includes an opening through one of the bucket portions on one side of said slit and a threaded opening in the other bucket portion on the other side of said slit, and a threaded member slidable through the said through opening and having means engaging the said one bucket portion, and threadedly engaging the said threaded opening in the other bucket portion on the other side of said slit.

References Cited in the file of this patent UNITED STATES PATENTS 2,394,124- Warren Feb. 5, 1946 FOREIGN PATENTS 586,836 Great Britain Apr. 2, 1947 609,446 Great Britain Sept. 30, 1948 668,093 Great Britain Mar. 12, 1 952 719,079 Great Britain Nov. 24, 1954 

